Duplex stainless steel alloy and use of this alloy

ABSTRACT

The present invention relates to a duplex stainless steel alloy containing in weight-%: C max 0.03%, Si&lt;0.30%, Mn 0-3.0%, P max 0.030%, S max 0.050%, Cr 25-29%, Ni 5-9%, Mo 4.5-8%, W 0-3%, Cu 0-2%, Co 0-3%, Ti 0-2%, Al 0-0.05%, B 0-0.01%, Ca 0-0.01%, and N 0.35-0.60%, balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume-%, and wherein each weight-% of Mo above may optionally be replaced by two (2) weight-% W.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a duplex stainless steel alloy, whichis a steel alloy having a ferritic-austenitic matrix and especially highresistance to corrosion in combination with good structural stabilityand hotworkability. The ferrite content is 30-70 volume-% and such steelalloys have a well balanced composition, which imparts the materialcorrosion properties, which make it suitable for use in for instancechloride-containing environments, such as in the sea.

BACKGROUND OF THE INVENTION AND PRIOR ART

Over the recent years, the environments in which corrosion resistantmetallic materials were used became more aggressive and the requirementson the corrosion properties as well as on their mechanical propertiesincreased. Duplex steel alloys, which were established as an alternativeto other steel grades used until then, for example high alloyedaustenitic steels, nickel-base alloys or other high alloyed steels, werealso a part of that development. An established measure for thecorrosion resistance in chloride-containing environments is theso-called Pitting Resistance Equivalent (abbreviated PRE), which isdefined as

PRE=% Cr+3.3% Mo+16% N

where the percentage for each element allude to weight-percent. A highernumerical value indicates a better corrosion resistance in particularagainst pitting corrosion. The essential alloying elements, which affectthis property, are according to the formula Cr, Mo, N. An example forsuch a steel grade is evident from EP0220141, which hereby through thisreference is included in this description. This steel grade with thedenotation SAF2507 (UNS 532750) was mainly alloyed with high contents ofCr, Mo and N. It is consequently developed against this property withabove all good resistance to corrosion in chloride environments.

In recent times also the elements Cu and W have shown to be efficientalloying additions for further optimization of the steel's corrosionproperties in chloride environments. The element W has by then been usedas substitute for a portion of Mo, as for example in the commercialalloy DP3W (UNS S39274) or Zeron100, which contain 2.0% respectively0.7% W. The latter contains even 0.7% Cu with the purpose to increasethe corrosion resistance of the alloy in acid environments.

The alloying addition of tungsten led to a further development of themeasure for the corrosion resistance and thereby the PRE-formula to thePREW-formula, which also makes the relationship between the influence ofMo and W on the alloys corrosion resistance clearer:

PREW=% Cr+3.3(% Mo+0.5% W)+16% N,

such as described for example in EP 0 545 753. This publication refersto a duplex stainless alloy with generally improved corrosionproperties.

The above-described steel grades have a PRE/PREW-number, irrespectivemethod of calculation, which lies above 40.

From the alloys with good corrosion resistance in chloride environmentsalso SAF 2906 shall be mentioned, which composition appears from EP 0708 845. This alloy, which is characterized by higher contents of Cr andN compared to for example SAF2507, has shown being especially suitablefor use in environments, where resistance to intergranular corrosion andcorrosion in ammonium carbamate is of importance, but it has also a highcorrosion resistance in chloride-containing environments.

U.S. Pat. No. 4,985,091 describes an alloy intended for use inhydrochloric and sulfuric acid environments, where mainly intergranularcorrosion arises. It is primarily intended as alternative to recentlyused austenitic steels.

U.S. Pat. No. 6,048,413 describes a duplex stainless alloy asalternative to austenitic stainless steels, intended for use inchloride-containing environments.

EP 0 683 241 discloses a duplex stainless steel alloy having acomposition resulting in improved properties with respect to resistanceto both stress corrosion cracking and pitting in chlorideion-containingenvironments than most other duplex stainless steel alloys known.However, this alloy as well as the alloys discussed above is highlysusceptible to intermetallic precipitation, especially sigma phaseprecipitation, which makes the material hard and brittle. Accordingly,the production of a material with good ductility by use of the duplexstainless steel alloy according to EP 0 683 241 is made very difficult.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a duplex stainlesssteel alloy of the type defined above and especially in the Europeanpatent 0 683 241, which has improved properties, especially ductilityand toughness, with respect to such an alloy already known whilemaintaining at least similar levels of corrosion resistance as such analloy. The alloy should have a good hotworkability.

This object is according to the invention obtained by providing a duplexstainless steel alloy, which contains in weight-%: C max 0.03%,Si<0.30%, Mn 0-3.0%, P max 0.030%, S max 0.050%, Cr 25-29%, Ni 5-9%, Mo4.5-8%, W 0-3%, Cu 0-2%, Co 0-3%, Ti 0-2%, Al 0-0.05%, B 0-0.01%, Ca0-0.01%, and N 0.35-0.60%, balance Fe and normal occurring impurities,wherein the ferrite content is 30-70 volume-%, and wherein each weight-%of Mo above may optionally be replaced by two (2) weight-% W.

It has been found that a duplex stainless steel alloy with thiscomposition has especially an increased ductility and toughness withrespect to the alloy according to EP 0 683 241, and it has also anincreased corrosion resistance. By reducing the Si content to be below0.30 weight-% a significant reduction in sigma phase precipitation isachieved, which is the key to the increased ductility and toughness ofthe steel alloy according to the invention. Thus, it has been found thatwhen using a comparatively high content of Mo it is highly efficient toreduce the content of Si for reducing the risk for intermetallicprecipitations.

According to an embodiment of the invention the content of Si is max0.25 weight-%, which makes the steel alloy even less prone tosigma-formation for increasing the ductility and toughness of thematerial. It is expected that the same would be valid if Molybdenumwould be partly or entirely replaced by Tungsten.

According to another embodiment of the invention the content of Si ismax 0.23 weight-%.

According to another embodiment of the invention the content of Mo is aweight-% and the content of W is b weight-%, wherein a+b/2>5.0. Such ahigh content of Mo and/or W results in excellent resistance tocorrosion, especially pitting- and crevice corrosion, but the increasedrisk for intermetallic precipitations with such high contents of theseelements is efficiently counteracted by the combination thereof with thelow content of Si. According to another embodiment of the inventiona>5.0. It is pointed out that claim 1 is to be interpreted as whenstarting from the content intervals of Mo (4.5-8%) and W (0-3%) it ispossible to replace each % of Mo by 2% of W or conversely, so that thecontent of Mo may for example be 3% when the content of W is at least3%. According to a preferred embodiment a+b/28, i.e. the total contentof Mo and W does not exceed 8%, for keeping the costs thereof at areasonable level. According to another preferred embodiment b=0, i.e.the alloy contains only Mo.

According to yet another embodiment of the invention the content of Cois 0-0.010 weight.-%. Co is an expensive material, and it has been foundthat the structure's ability as well as the corrosion resistanceimprovement influence thereof is not an essential factor in a steelalloy with a composition according to the present invention.

According to another embodiment of the invention the content of ferriteis 40-60 volume-%.

According to another embodiment of the invention the average PRE- orPREW-value of the two phases of the alloy exceeds 44, whereby PRE=%Cr+3.3% Mo+16% N and PREW=% Cr+3.3(% Mo+0.5% W)+16% N, wherein % isweight-%. The PRE- or PREW-value for both the ferrite and austenitephase may be higher than 47, preferably higher than 48.5, and saidaverage PRE- or PREW-value may be higher than 48, preferably higher than49. It has turned out that the pitting and crevice corrosion resistancein the steel alloy according to the invention is especially increased byincreasing the PRE- or PREW-value of the phase with the lowest suchvalue. It has been found that the steel alloy according to the inventionwill still have a good hotworkability with a PRE- or PREW-value higherthan 49.

According to another embodiment of the invention the ratio betweenPRE(W)-value for the austenite phase and PRE(W)-value for the ferritephase lies between 0.90 and 1.15, preferably between 0.95 and 1.05.

An alloy according to the present invention is suitable to be used inchloride-containing environments in product forms such as bars, tubes,such as welded and seamless tubes, plate, strip, wire, welding wire,constructive parts, such as for example pump, valves, flanges andcouplings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a calculated phase content of a duplex stainless steelalloy according to an embodiment of the invention as a function oftemperature,

FIG. 2 is a graph similar to FIG. 1 for a reference steel alloyaccording to EP 0 683 241, and

FIG. 3 is a micrograph of continuously cooled samples of the alloysaccording to FIG. 1 and FIG. 2 according to three different coolingspeeds.

DETAILED DESCRIPTION OF THE INVENTION

Good corrosion resistance properties as well a high ductility andtoughness is obtained by the combination of elements in a duplexstainless steel alloy according to the invention. This steel alloy hasalso good workability, which enables for example extrusion to seamlesstubes. The alloy according to the invention contains (in weight-%):

C max 0.03% Si <0.30% Mn   0-3.0% P max 0.030% S max 0.050% Cr 25-29% Ni5-9% Mo 4.5-8%   W 0-3% Cu 0-2% Co 0-3% Ti 0-2% Al   0-0.05% B   0-0.01%Ca   0-0.01% N 0.35-0.60%balance Fe and normal occurring impurities, wherein the ferrite contentis 30-70 volume-%, and wherein each weight-% of Mo above may optionallybe replaced by two (2) weight-% W.

Carbon (C) has limited solubility in both ferrite and austenite. Thelimited solubility implies a risk of precipitation of chromium carbidesand the content should therefore not exceed 0.03 weight-%, preferablynot exceed 0.02 weight-%.

Silicon (Si) is utilized as desoxidation agent in the steel productionand it increases the flowability during production and welding. However,too high contents of Si lead to precipitation of unwanted intermetallicphase, wherefore the content is limited to below 0.30 weight-%,preferably max 0.25 weight-%, more preferably max 0.23 weight-%.

Manganese (Mn) is added in order to increase the N-solubility in thematerial. However, it has shown that Mn only has a limited influence onthe N-solubility in the type of alloy in question. Instead there arefound other elements with higher influence on the solubility. Besides,Mn in combination with high contents of sulfur can give rise toformation of manganese sulfides, which act as initiation-points forpitting corrosion. The content of Mn should therefore be limited tobetween 0-3.0 weight-%, preferably 0.5-1.2 weight-%.

Phosphorus (P) is a common impurity element. If present in amountsgreater than approximately 0.05%, it can result in adverse effects one.g. hot ductility, weldability and corrosion resistance. The amount ofP in the alloy should therefore not exceed 0.05%.

Sulfur (S) influences the corrosion resistance negatively by formingsoluble sulfides. Furthermore, the hotworkability deteriorates, for whatreason the content of sulfur is limited to max 0.030 weight-%,preferably less than 0.010 weight-%.

Chromium (Cr) is a much active element in order to improve theresistance to a majority of corrosion types. Furthermore, a high contentof chromium implies that one gets a very good N-solubility in thematerial. Thus, it is desirable to keep the Cr-content as high aspossible in order to improve the corrosion resistance. For very goodamounts of corrosion resistance the content of chromium should be atleast 25 weight-%. However, high contents of Cr increase the risk forintermetallic precipitations, for what reason the content of chromiummust be limited up to max 29 weight-%, preferably 25.5-28 weight-%.

Nickel (Ni) is used as austenite stabilizing element and is added insuitable contents in order to obtain the desired content of ferrite. Inorder to obtain the desired relationship between the austenitic and theferritic phase with between 30-70 volume-% ferrite, an addition of 5-9weight-% nickel is required, and it is preferably 6-8 weight-%.

Molybdenum (Mo) is an active element which improves the resistance tocorrosion in chloride environments as well as preferably in reducingacids. A too high Mo-content in combination with high Cr-contents,implies that the risk for intermetallic precipitations increases. TheMo-content in the present invention should lie in the range of 4.5-8weight-%, preferably above 5.0 weight-%, in which each weight-% of Momay optionally be replaced by 2 weight-% W.

Tungsten (W) increases the resistance to pitting- and crevice corrosion.But the addition of too high contents of tungsten in combination withthat the Cr-contents as well as Mo-contents are high, means that therisk for intermetallic precipitations increases. The W-content in thepresent invention should lie in the range of 0-3.0 weight-%.

Copper (Cu) may be added in order to improve the general corrosionresistance in acid environments such as sulfuric acid. At the same timeCu influences the structural stability. However, thigh contents of Cuimply that the solid solubility will be exceeded. Therefore theCu-content should be limited to max 2.0 weight-%, preferably between 0and 1.5 weight-%, more preferred 0.1-0.5 weight-%.

Cobalt (Co) has properties that are intermediate between those of ironand nickel. Therefore, a minor replacement of these elements with Co, orthe use of Co-containing raw materials (Ni scrap metal usually containssome Co, in some cases in quantities greater than 10%) will not resultin any major change in properties. Co can be used to replace some Ni asan austenite-stabilizing element. Co is a relatively expensive element,so the addition of Co is limited to be within the range of 0-3 weight-%.

Titanium (Ti) has a high affinity for N. It can therefore be used e.g.to increase the solubility of N in the melt and to avoid the formationof nitrogen bubbles during casting. However, excessive amounts of Ti inthe material causes precipitation of nitrides during casting, which candisrupt the casting process and the formed nitrides can act as defectscausing reduction in corrosion resistance, toughness and ductility.Therefore, the addition of Ti is limited to 2 weight-%.

Aluminium (AI) and Calcium (Ca) are used as desoxidation agents at thesteel production. The content of Al should be limited to max 0.05weight-%, preferably max 0.03%, in order to limit the forming ofnitrides. Ca has a favourable effect on the hotductility. However, theCa-content should be limited to max 0.010 weight-% in order to avoid anunwanted amount of slag.

Boron (B) may be added in order to increase the hotworkability of thematerial. At a too high content of Boron the weldability as well as thecorrosion resistance could deteriorate. Therefore, the content of boronshould be limited to max 0.01 weight-%.

Nitrogen (N) is a very active element, which increases the corrosionresistance, the structural stability as well as the strength of thematerial. Furthermore, a high N-content improves the recovering of theaustenite after welding, which gives good properties within the weldedjoint. In order to obtain a good effect of N, at least 0.35 weight-% Nshould be added. At high contents of N the risk for precipitation ofchromium nitrides increases, especially when simultaneously the chromiumcontent is high. Furthermore, a high N-content implies that the risk forporosity increases because of the exceeded solubility of N in the smelt.For these reasons the N-content should be limited to max 0.60 weight-%,preferably >0.35-0.45 weight-% N is added.

The content of ferrite is important in order to obtain good mechanicalproperties and corrosion properties as well as good weldability. From acorrosion point of view and a point of view of weldability a content offerrite between 30-70% is desirable in order to obtain good properties.Furthermore, high contents of ferrite imply that the impact strength atlow temperatures as well as the resistance to hydrogen-inducedbrittleness risks deteriorating. The content of ferrite is therefore30-70 volume-%, preferably 40-60 volume-%.

Description of a Preferred Embodiment

Two experimental alloys were produced in order to test primarily theeffect of different concentrations of Si. Table 1 below shows thecontent of the two alloys No. 1 and No. 2, in which No. 1 is a duplexstainless steel alloy according to an embodiment of the presentinvention and alloy No. 2 is such an alloy according to EP 0683241.

TABLE 1 Alloy No. 1 2 C 0.017 0.019 Si 0.21 0.62 Mn 0.49 0.47 P 0.0050.004 S 0.006 0.008 Cr 26.06 26.10 Ni 7.11 7.03 Mo 5.20 5.16 W <0.01<0.01 Cu <0.01 0.021 Co <0.010 <0.010 Ti <0.005 <0.005 Al 0.004 0.007 B24 ppm 25 ppm Ca 22 ppm 28 ppm N 0.41 0.42

Furthermore, the alloys were modelled using the Thermo-Calc softwarewith database CCTSS (a slightly modified version of the commercialdatabase TCFE3 with improved models for e.g. duplex alloy compositions).FIGS. 1 and 2 show the calculated phase contents of alloy No. 1 andalloy No. 2, respectively, as a function of the temperature. In theseFigures:

-   1: The ferrite content. It is seen that for the alloy according to    the present invention (FIG. 1) a heat treatment in the region of 1    100-1 300° C. is needed for obtaining a ferrite content desired-   2: The austenite content. The heat treatment is carried out so that    only a ferrite phase and an austenite phase are obtained.-   3: The content of N-   4: Liquid metal-   5: Sigma-phase. The formation thereof may be avoided by rapid    cooling.-   6: Content of Cr₂N, which cause brittleness and reduction in    corrosion resistance.-   7: Carbide content, which should be kept low for not influencing    welds. A high tendency for carbide precipitation leads to the risk    of reduced corrosion resistance near welds. The equilibrium amount    of carbides should therefore be kept low.-   8: Intermetallic phase. The sum of this and the sigma phase is to be    kept as low as possible.

TABLE 2 PRE_(γ)/ PRE_(α) PREγ PRE_(α) Precipitates Alloy at at at % α atpresent at No. PRE 1100° C. 1100° C. 1100° C. T_(max,σ) T_(max,) Cr₂N1100° C. 1100° C. 1 49.8 49.1 50.3 1.02 1078 1043 1018 43.3 2 49.8 48.350.0 1.04 1037 1108 1108 47.1 0.3 wt/% Cr₂N

Table 2 above shows the total PRE of the two alloys and the predictedPRE for each phase when quenched from 1100° C., as well as the ratiobetween PRE in the austenite and in the ferrite. It also shows thepredicted ferrite content after a quench from 1100° C. and finally thepredicted dissolution temperatures for Cr₂N and sigma (σ) phase, and thepredicted presence of any precipitates at 1100° C. Since theprecipitation of Cr₂N is more rapid than that of a phase, two T_(max),Cr₂N are presented, one for the case for slow cooling when equilibriumamounts of σ are allowed to precipitate (“with σ”) and another for rapidcooling when a does not precipitate (“without s”). It is clear that bothalloys fulfil the requirements on ferrite content, total PRE as well asPRE balance and minimum PRE in each phase as stated in our WO 03020994.

Sample Manufacturing

The alloys were produced by melting, casting of ingots and finally pressforging. Table 3 shows the results of the forging. The forging wasinterrupted when severe surface defects began to form, and the totalreduction of cross-sectional area during the forging process can thus beused as an estimate of the forgeability of the two alloys.

TABLE 3 Relative Start Finished Area Area reduction Alloy No. DimensionDimension (B) A/B (1 − B/A) * 100 1 230 × 230 mm  85 × 85 mm 7.3 86% 2230 × 230 mm 125 × 125 mm 3.4 70%

The forged bars were annealed at 1100° C., followed by quenching inwater before any further processing was begun. The prematerial used forsamples was annealed once more, after sectioning into smaller pieces, at1100° C. for 1 h, followed by water quench. After this treatment, thedifferent samples were machined.

Testing Impact Testing

Impact testing was performed on 10×10 mm Charpy v-notch samples (55 mmlong) in four different materials conditions: asannealed (i.e. 1100°C./water quench) and with an additional anneal of the impact samples ata lower temperature. Table 4 shows the different materials conditions aswell as the resulting impact toughness values. Two samples were testedfor each composition and annealing condition.

TABLE 4 1100/wq + 1100/wq + 1100/wq + 1025/ Alloy No. 1100/wq 1075/wq1050 wq wq 1 high Mo, 175, 176 232, 240 26, 28 6, 8 low Si 2 high Mo,168, 154 150, 178 14, 10 5, 4 high Si

Alloy 1, with a high Mo content and low Si and Co contents has a goodimpact toughness provided a sufficiently high annealing temperature isused. This Table illustrates a weakness of the alloy 2 according to EP 0683 241, namely that a Si content higher than 0.5% together with a highMo content gives a potentially brittle material. Just reducing the Sicontent (as in the alloy 1 according to the present invention) gives alarge improvement in toughness.

Continuous Cooling

9 samples from each heat was annealed at 1100° C. and then reheated tothree different temperatures: 1050, 1100 and 1150° C. from each heat,respectively. The samples were cooled at three different constantcooling rates from the different holding temperatures: 20, 60 and 140°C./min. This means that 9 different annealing cycles were used for eachheat. No nitrides were found in any of the samples. Table 5 summarisesobservations made by optical microscopy. A relative ranking index isused for the σ phase content of different samples, where:

0: no σ phase detected1:1-2 σ phase particles on average detected in a field of view at 500×magnification2: small amounts of σ phase detected at 500× magnification (but morethan 2 particles/field of view)3: relatively large amounts of σ, but with less than 5% of ferritetransformed4: more than 5% of the ferrite transformed to σ5. more than 25% of the ferrite transformed to σ6: more than 50% of the ferrite transformed to σ

TABLE 5 Heating cycles Alloy number Heating temperature Cooling rate 1 21050° C.  20° C./min 6 6  60° C./min 4 4 140° C./min 2 3 1100° C.  20°C./min 4 5  60° C./min 2 3 140° C./min 1 2 1150° C.  20° C./min 4 5  60°C./min 2 3 140° C./min 2 2

It is shown that alloy 1 is slightly less prone to a precipitation thanalloy 2. It is pointed out that a “note” of 2, preferably 1, isnecessary for making it possible to properly manufacture the material inquestion.

FIG. 3 shows micrographs of the continuously cooled samples heated to1100° C. Light colour is austenite, brown is ferrite and blackish isσ-phase. It is shown that the formation of σ-phase (blackish) isremarkably weaker for alloy No. 1 according to the present inventionthan for alloy No. 2 according to EP 0 683 241, which is obviously dueto the lower content of Si.

Mechanical Properties

Table 6 shows results from tensile testing. Alloy No. 2 is apparentlyless ductile than alloy No. 1 according to the invention.

TABLE 6 Results from tensile testing. Two samples from each heatUltimate tensile Yield strength, strength, Reduction of AlloyR_(p0,2)/MPa R_(m)/MPa Elongation/% area/% 1 644, 626 841, 844 37.9,37.5 61, 60 2 687 847 17.0 27

Corrosion Testing

Critical crevice temperature (CCT) according to MTI-2 and criticalpitting temperature (CPT) in “Green Death” solution (1% FeCl₃+1%CuCl₂+11% H₂SO₄+1.2% HCl) is shown in Table 7. There is very littledifference in crevice corrosion resistance between the different alloys.The assumption that pitting and crevice corrosion resistance in duplexalloy is mainly determined by the PRE of the phase with lowest PREagrees with the fact that alloy 1 has the highest CCT. Furthermore,improved behaviour of Alloy 1 with respect to Alloy 2 appears in theform of a lower weight loss due to corrosion and higher maximumtemperatures.

TABLE 7 Results from crevice corrosion testing according to MTI-2,pitting corrosion in Green Death solution and pitting corrosion inferric chloride. Two samples/alloys were used for each test. CPT (° C.),in ferric G48 A test at CPT (° C.), in chloride, modified G48C 95° C.,PRE in Alloy CCT (° C.), Green Death (average weight loss (average“weakest” No. MTI-2 solution after 97.5° C./g) weight loss/g) phase 165, 70 80, 80 97.5, 97.5 (0.0036) No pits (0.014) 49.1 2 60, 65 70, 7597.5, 97.5 (0.011) Small pits 48.3 (0.04)

SUMMARY

The alloy (No. 2) corresponding to EP 0 683 241 is highly susceptible tosigma phase precipitation, which makes the production of a material withgood ductility very difficult. This problem is solved by lowering the Sicontent and a good balance between the PRE-values of the two phases.Furthermore, the alloy No. 2 has a low forgeability. By reducing the Sicontent of an alloy of the type defined in EP 0 683 241, i.e. by using acomposition of alloy No. 1, not only will the ductility and toughnessincrease, the corrosion resistance is increased as well, which in factis an effect that was quite unexpected.

1. A duplex stainless steel alloy, comprising, in weight-%: C max 0.03%Si <0.30% Mn   0-3.0% P max 0.030% S max 0.050% Cr 25-29% Ni 5-9% Mo4.5-8%   W 0-3% Cu 0-2% Co 0-3% Ti 0-2% Al   0-0.05% B   0-0.01% Ca  0-0.01% N 0.35-0.60%

balance Fe and normal occurring impurities, wherein the ferrite contentis 30-70 volume-%, and wherein each weight-% of Mo may optionally bereplaced by two weight-% W.
 2. Alloy according to claim 1, wherein thecontent of Si is max 0.25 weight-%.
 3. Alloy according to claim 1,wherein the content of Si is max 0.23 weight-%.
 4. Alloy according toclaim 1, wherein the content of Mo is a weight-% and the content of W isb weight-%, wherein a+b/2>5.0.
 5. Alloy according to claim 4, whereina >5.0.
 6. Alloy according to claim 4, wherein a+b/2≦8.
 7. Alloyaccording to claim 1, wherein the content of Co is 0-0.010 weight-%. 8.Alloy according to claim 1, wherein the content of Cr is 25.5-28weight-%.
 9. Alloy according to claim 1, wherein the content of Ni is6-8 weight-%.
 10. Alloy according to claim 1, wherein the content of Nis 0.35-0.45 weight-%.
 11. Alloy according to claim 1, wherein thecontent of ferrite is 40-60 volume-%.
 12. Alloy according to claim 1,wherein an average PRE- or PREW-value of the two phases of the alloyexceeds 44, where PRE=% Cr+3.3% Mo+16% N and PREW=% Cr+3.3(% Mo+0.5%W)+16% N, and where % is weight-%.
 13. Alloy according to claim 12,wherein a PRE- or PREW-value for both the ferrite and the austenitephase is higher than 47 and that said average PRE- or PREW-value ishigher than
 48. 14. Alloy according to claim 12, wherein a ratio betweenPRE(W)-value for the austenite phase and PRE(W)-value for the ferritephase lies between 0.90 and 1.15.
 15. Use of an alloy according to claim1 in chloride containing environments in product forms such as bars,tubes, such as welded and seamless tubes, plate, strip, wire, weldingwire, constructive parts, such as for example pumps, valves, flanges andcouplings.
 16. A method to improve corrosion resistance in a productexposed to a chloride containing environment, the method comprisingforming the product from an alloy according to claim
 1. 17. The methodof claim 16, wherein the product is a bar, a welded tube, a seamlesstube, a plate, a strip, a wire, a welding wire, a pump, a valve, aflange or a coupling.
 18. Alloy according to claim 13, wherein the PRE-or PREW-value for both the ferrite and the austenite phase is higherthan 48.5
 19. Alloy according to claim 13, wherein the average PRE- orPREW-value is higher than
 49. 20. Alloy according to claim 14 whereinthe ratio between PRE(W)-value for the austenite phase and PRE(W)-valuefor the ferrite phase lies between 0.95 and 1.05.